With the progress of computer image processing techniques, a method of directing recording images on a photosensitive layer by light irradiation corresponding to digital signals has recently been developed and thus an intense interest has been shown toward a computer-to-plate (CTP) system in which images are directly formed on a photosensitive lithographic printing plate, without outputting onto a silver salt mask film, by employing the method in a lithographic printing plate precursor. The CTP system, which uses a high-output laser having a maximum intensity within a near infrared or infrared range as a light source for light irradiation, has the following advantages: images having high resolution can be obtained by exposure within a short time and the photosensitive lithographic printing plate used in the system can be handled in daylight. Regarding solid and semiconductor lasers capable of emitting infrared ray having a wavelength of 760 to 1200 nm, a high-output and portable laser is available with ease.
By the way, as a lithographic printing plate precursor which can form images using solid laser or semiconductor laser, there has been proposed a lithographic-printing plate precursor comprising a substrate, an image recording layer formed on the substrate, and protective layer of the image recording layer so as to prevent scratching of the surface of the lithographic printing plate precursor.
In Japanese Unexamined Patent Publication (Kokai) No. 2004-157459, for example, a lithographic-printing plate precursor comprising a lower layer containing a water-insoluble and alkali-soluble polyurethane resin and an upper layer containing a m, p-cresol novolak resin is described and this lithographic printing plate precursor is excellent in printing durability and press life. However, this lithographic printing plate precursor has a problem in that it has narrow development latitude in a developing solution having the pH of 11 or lower, and also the upper layer is peeled off during the development to form deposits on a developing tank, and thus there is room for improvement in its developing properties.